1. Field of the Invention
Aspects of the invention relate to an image forming apparatus. More particularly, aspects of the invention relate to an image forming apparatus which permits adjustment of a gap between a photosensitive drum and a developing roller.
2. Description of the Related Art
Generally, an electro-photographic image forming apparatus, such as a laser beam printer, produces a desired image by forming an electrostatic latent image on a photosensitive medium such as a photosensitive drum or photosensitive belt, developing the formed electrostatic latent image using developing materials having predetermined colors, and transferring the developed image to a recording medium. A developing unit of the image forming apparatus includes components, such as a photosensitive drum, a developing roller, a supplying roller and an agitating roller, and is adapted to be handled as a unit. These rotating rollers rotate in predetermined speed ratios and directions through a series of gears.
The developing unit of the image forming apparatus is typically commanded to produce a test image for inspection after manufacture, and is packaged and shipped, if the developing unit passes the inspection. However, a developing material at a development nip, where the photosensitive drum and developing roller contacts with each other, can be fused on the surface of the photosensitive drum or the surface of the developing roller as a result of high temperature and pressure. If the developing material is fused on the photosensitive drum, the developing material cannot be replenished to the surface of the photosensitive drum for latent image development, and a white image can be produced on a recording medium.
Alternatively, if the developing material is fused on the developing roller, as the photosensitive drum and developing roller do not rotate at a rate or ratio of 1:1, the fused developing material is usually removed in small amounts. However, between 10 and 100 or several hundred sheets of recording media typically have to be printed until the fused developing material is completely removed.
In addition to the above problem of the fused developing material, after the developing unit is sealed in a packing bag, further sealed in a packing box, and shipped, the temperature of the developing unit can rise depending upon the surrounding environment. Also, low molecular weight elements can leak out from particles constituting the developing roller, a charge roller or the photosensitive drum (what is called a migration phenomenon), or the rollers can be squeezed. These problems can be more serious in a contact development method, where the photosensitive drum and the developing roller form a development nip. Also, a band can be formed at the development nip, which can cause production of a defective image having a corresponding blackened or whitened portion. In this regard, a conventional solution is to adopt an extra member to separate the photosensitive drum from the developing roller during a distribution process of the developing unit.
FIG. 1 is a diagram showing the structure of a developing unit having a conventional gap setting device. Referring to FIG. 1, the gap setting device 5 is inserted between a photosensitive drum 1 and a developing roller 3. The gap setting device 5 can be made of molded goods, metals, paper or sheets. However, a problem can occur in that the user in person must usually remove the gap setting device 5.
An example to maintain the gap between a photosensitive drum and developing roller is disclosed in Japanese Patent Laid-Open Publication No. 1997-204101. According to Japanese Patent Laid-Open Publication No. 1997-204101, in a developing unit, a support plate to support the shaft of the developing roller is placed to be freely slidable relative to the sliding axis thereof, a coil is placed on the support plate, and the coil is driven through a magnetic field created by a magnet and a yoke. Further, to keep constant the gap between the developing roller and the photosensitive drum, the gap is measured through laser emitting and receiving devices, an electric current is supplied to the coil, and the support plate is slid relative to the sliding axis, moving the developing roller in conjunction with the support plate.
Another example to adequately maintain the gap between a photosensitive drum and developing roller is disclosed in U.S. Pat. No. 6,385,417. According to U.S. Pat. No. 6,385,417, in a developing unit, the photosensitive drum is provided to form an electrostatic latent image on the surface thereof, the developing roller is provided to develop the electrostatic latent image using a developing material, and a gap regulating unit is provided to maintain constant the gap between the photosensitive drum and developing roller. The gap regulating unit includes first bearings installed at both ends of a shaft of the photosensitive drum, and second bearings installed at both ends of a shaft of the developing roller.
The disclosed developing units in Japanese Patent Laid-Open Publication No. 1997-204101 and U.S. Pat. No. 6,385,417 are structured to maintain constant the gap between the photosensitive drum and developing roller in non-contact development methods. Thus, various problems in the distribution process of a developing unit of an image forming apparatus are not directly considered in these developing units disclosed in Japanese Patent Laid-Open Publication No. 1997-204101 and U.S. Pat. No. 6,385,417.